Optical fiber is a common transmission medium for telecommunications. It is especially advantageous for long distance communications because light propagates through the fiber with little attenuation as compared to electrical cables and because higher data rates are possible.
In most long distance communication scenarios, single-mode fiber is used. Single-mode fiber typically has a core diameter in the range of approximately 8-10 microns (μm) and can only support a single spatial mode, or pathway, for light signals. Although multi-mode fibers can support more modes (typically 100-200 modes) and therefore could be used to transmit more data, multi-mode fiber suffers from distortion issues, such as modal dispersion, which become particularly problematic over longer distances.
Despite providing advantages over multi-mode fibers in long distance communication scenarios, single-mode fibers suffer from nonlinearity problems, such as self-phase modulation (SPM), cross-phase modulation (XPM), and four-wave mixing (FWM). Those phenomena are major limitations on fiber transmission capacity. Significant efforts have been made to reduce, mitigate, or remove nonlinear penalties. Such efforts include using dispersion maps, using more nonlinearity-tolerant modulation formats such as differential phase-shift keying (DPSK), using amplification schemes such as Raman amplification, designing fibers with larger effective area, and compensating nonlinear impairments using digital signal processing (DSP) techniques. Unfortunately, all of those methods are limited in the extent to which they can reduce the nonlinear impairments.
In view of the nonlinearity issues associated with single-mode fibers and the difficulty in compensating for them, it can be appreciated that it would be desirable to have alternative means for optically transmitting information, particularly across long distances.